Marine vessels (e.g., offshore platforms, drilling vessels, or production vessels) often include independent power systems that have a generator coupled to a source of mechanical energy (e.g., a diesel engine or a gas turbine). Electric power produced by the generator is distributed over a power distribution bus that is connected to a plurality of consumers. Large consumers include thruster drives, a drilling drive, or draw works. Smaller consumers may be pumps, compressors, and other electric devices provided on a marine vessel.
Unlike onshore power systems that may derive electric energy from a plurality of sources, the isolated power systems of marine vessels have a relatively low number of generators installed (e.g., 4 to 6). Due to the short cable routes, the use of bus bars and the absence of transformers in the path, the impedances between the generators may be low (e.g., almost insignificant). If there is a fault in the power system (e.g., a bus bar fault or a generator interconnection cable fault), a high voltage dip may occur due to the low impedances. Electric motors connected to the power system may operate under a significantly reduced current supply and, as a result, slow down. For example, the power system of the marine vessel may have tens or hundreds of small electric motors (e.g., direct online or DOL induction motors) connected.
After the fault is cleared, the electric motors will increase speed back to nominal, thereby resulting in a significant inrush current. In some systems, an automatic voltage regulator (AVR) controls the generator and, therefore, the voltage on the main power distribution bus. During system recovery after the fault, a voltage overshoot may occur as the AVR tries to compensate for the voltage drop. The overshooting may be higher than 150%. If the marine vessel is in transit, the marine vessel may operate in a DP2 mode (e.g., according to dynamic positioning 2 specifications of Det Norske Veritas (DNV)). In the DP2 mode of operation, the voltage overshot may not be a problem. If the overshoot leads to a blackout of another component that is non-essential to DP2 operation, the component may be isolated from the system and repaired. On the other hand, if the vessel operates in a DP3 mode (e.g., when performing a drilling operation), the bus tie breakers of the power distribution bus are open, and the power system of the marine vessel is split into several independent power systems. For example, two, three or four sections of the power distribution bus may be provided. Each of the sections operates as an independent redundant power system. If there is a fault in one of the subsystems, the other subsystems will not be affected since the other subsystems are electrically isolated from the faulty subsystem. Thus, a voltage overshoot that may occur after resuming operation of a faulty subsystem will not be experienced by the remaining subsystems.
Although operating the power system with open bus tie breakers provides redundancy and avoids a complete blackout in the case of a fault in a component, there is a disadvantage in that a generator is operated in each subsection, and the operation of the generators may be ineffective. Some generators may operate at maximum capacity while other generators may operate only close to idle.